One of the well-known production systems utilizes a dynamically positioned offshore vessel fitted with a derrick and a riser constructed of drill pipe threaded joints and the riser's stability is provided by a tension applied to the top-end of the riser by a vessel tensioning system, which is located beneath the derrick. This production system has high operational costs because it utilizes a vessel that is not easily available.
The use of free standing riser is also known in production as well as in completion systems. For example, U.S. Pat. No. 4,234,047 (hereinafter the '047 reference) describes the use of a free standing drilling riser utilizing inflatable buoys installed in the upper-end of the riser. This system permits a quick disconnection of the floating vessel and the riser, which remains buoyantly in place on the sea bed, in a vertical position. Although the specification of the '047 reference does not explicitly address this technical aspect, the use of a rig vessel and a compensator are necessary for the handling of the upper section of the riser, as may be seen in the figures accompanying this reference.
A free standing riser including various annular chambers that control buoyancy is described in U.S. Pat. No. 4,646,840 (hereinafter the '840 reference). However, only anchored vessels may be used with this system since there is no swivel device for the riser. Thus, the arrangement described in the '840 reference provides little practicability for lowering a WCT utilizing the same production riser.
U.S. Pat. No. 4,762,180 describes a configuration with a wellhead, a riser, a riser tensioning buoy, and a WCT on top of the buoy, in that order. This configuration is not suitable for a Long Duration Test LDT since, after the referenced test, the resulting configuration does not include a typical arrangement of the equipment, namely a wellhead, a subsea WCT, a flowline supported at the seabed, and finally a riser in ascendant catenary to the FPU, in that order.
U.S. Pat. No. 5,046,896 (hereinafter the '896 reference) describes a riser with air filled buoys, instead of rigid buoys. The use of these air filled rigid buoys, although not directly addressed in the specification of the '896 reference, also requires a rig vessel and a compensator for handling the top section of the riser.
It is emphasized that, in all the free standing riser systems mentioned above, the technologies therein described require that the vessel be fitted with a derrick and compensator for handling the upper section of the riser (i.e., the section above the point of disconnection).
U.S. Pat. No. 6,082,391 and U.S. Pat. No. 6,321,844 describe a system for the conveyance of petroleum from the seabed in deep water to a floating structure at the surface, in which at least one rigid and straight riser is vertically positioned. This hybrid riser has a central rigid tubular structure and a cylindrical block of syntactic material that surrounds the rigid tubular structure. The cylindrical block provides both buoyancy and thermal insulation to the riser. A floating reservoir is provided above the riser. Multiple rigid pipes for receiving petroleum from the seabed are inserted in the syntactic material. Flexible pipes connect the rigid pipes to the floating structure. Thus, the rigid riser does not provide for passage through the floating reservoir. In addition, the riser, which includes multiple pipes and an insulation system, must be constructed and assembled at a dry location (i.e., on land). Once installed, its reutilization at a different water depth can be troublesome and quite limited, since the method of fabrication is by means of welding the joints.
FIG. 5 shows an example of a conventional free standing riser system. The stability of the system 200 is provided by the buoyancy of a buoy assembly 60 that is connected to the upper-end of the riser 50 by a tether 212. A flexible jumper 90 is connected to the end of a pipe 714 at the upper-end of the riser 50. The flexible jumper 90 is interconnected to an FPU. The lower-end 213 of the riser 50 is connected to a foundation 210 on the seabed. A spool 211 is used to connect the lower-end 213 to a pipe 214 installed on the seabed.
The system 200 in FIG. 5 requires the construction of a foundation 210, whose function is solely to anchor the riser 50 and to support the loads transmitted by the same.
Hence, in spite of the technological advances in the area, there is a continuing need for a free standing riser system including a riser formed of interconnected joints and being coupled at its lower-end to a subsea equipment, the riser being fitted with a subsea intervention unit. Such a system would provide easy access and maintenance of the well, allow easy installation and retrieval, and allow the system to be adapted to different water depths.